Cranes, such as gantry cranes are used for lifting and handling loads. In particular, gantry cranes are used to lift such as truck trailers, cargo containers, boats and the like. The cranes normally have a gantry structure that spans over the load(s). For example, in intermodal applications, the crane may span over two adjacent railroad cars, a truck trailer adjacent a railroad car or side-by-side stacks of containers. Gantry cranes are frequently self-mobile, moving on tracks or wheels.
Conventionally, some sort of apparatus, such as a lift frame or a lifting yoke, is suspended from the gantry structure to engage and lift loads. Such apparatus must be moveable at least up and down. For intermodal applications the apparatus is also preferably moveable side-to-side and may be tilted end-to-end and side-to-side.
Movement of the crane itself and its moving components is generally accomplished directly, or indirectly, by the operator using control circuitry and apparatus which is manually-actuated and articulated to achieve desired variable speeds and directions. Controls, such as, joysticks, manually-rotatable wheels or roller balls, foot pedals and the like are moved and positioned by an operator. The movement and position are translated into a control signal to move and position a given component of the crane itself.
These controls must provide a wide range of control. For example, to engage loads and to maneuver in limited spaces, the controls must provide for slow and careful movement of the crane and its components. On the other hand, because of the high duty cycles required for efficient commercial operations, the controls must also provide for higher speeds when such precision of movement is not required. Also, for speed and efficiency of operation, an operator must be able to quickly and continuously vary the speeds from low to high and any appropriate speeds in-between.
Accordingly, conventional control systems provide manual controls permitting a continuous range of vehicle and component movement speed from a minimum to maximum speed which is relatively broad. Thus, accuracy and consistency of speed and direction affected by the manual controls on the vehicle or a given component is a function of the physical acuity of the operator attempting to physically position the control appropriately between its maximum and minimum value.
This poses a problem during operations which require a great deal of very precise, or slow, movement such as encountered by operators when positioning the crane or its lifting apparatus for proper engagement with, or over, a load.
For example, when using a conventional joystick to control the crane or its other moveable components, the operator can repeatedly overshoot, or undershoot, the position desired to properly align a lifting apparatus over the load. This is particularly problematic when twist locks are involved where all four corners of a lifting apparatus must be aligned with all four upper comers of a container to be lifted. Using the mechanical range and variable response of a conventional joystick to properly align the twist locks can be difficult and time consuming.
Another example of an operation which requires precision of speed and direction of movement is found in marine applications where the crane wheels must be directed on narrow sea walls in order to position the crane over a boat in the water. Such sea walls are frequently not much wider than the wheels of the crane and a small control error by the operator could be disastrous.
The present invention is proposed to solve these problems and to provide other advantages not provided in the same manner by conventional apparatus.